Spectrometers are used for the spectral intensity distribution measurement of light and radiation. They are widely used in the fields of light source measurement, color measurement, chemical analysis and so on. Existing multi-channel array spectrometers have advantages of having much higher measurement speed over conventional scanning spectrometers. It takes only several milliseconds to complete a measurement. However, the existing multi-channel array spectrometers still have some disadvantages that cannot be easily resolved. Among these disadvantages, dynamic range limitation and stray light performance are the two major issues which bring challenges to designers and manufacturers.
A photoelectric device of a multi-channel array spectrometer is an array detector such as CCD or PDA. Pixels of the array detector detect the entire spectrum simultaneously, and convert them to electrical signals. The signal of each pixel is proportional to the intensity of light incident on it. After calibration against a standard source with known spectral power distribution, the spectrometer can measure the spectral power distributions of a test light source.
Integration of the spectral power distribution is radiometric quantity of the test light source in an integrated wavelength range. The integration of the spectral power distribution weighted with V(λ) function is photometric quantity of the test light source, where V(λ) function is the International Commission on Illumination (CIE) standard spectral luminous efficiency function. Thus, the radiometric quantity or photometric quantity of the test light source can be measured.
The photoelectric device of the multi-channel array spectrometer, CCD or PDA, has a narrow dynamic range. As a result, the responsivity curves of the photoelectric device have non-linear problems. The signals of the practical pixels are not strictly proportional to the intensity of the light incident on them. The non-linear problem also occurs along with the integration time. Thus, errors occur in measurement using existing multi-channel array spectrometer.
The radiometric or photometric quantity can also be measured by a broad-band radiometer or photometer detector: a signal from the detector of photoelectric device is proportional to the radiometric or photometric quantity of the test light source. The photoelectric device is always a silicon photodiode. The silicon photodiode can have good linearity in a wide dynamic range. Good silicon photodiodes can reach <0.2% in 8 orders of magnitude. However, the accuracy of radiometric or photometric measurement depends on the relative spectral responsivity of the detector. For a photometer detector, its relative spectral responsivity should precisely match V(λ). For a radiometer detector, its relative spectral responsivity should precisely match a flat straight line, that is, its spectral responsivity has the same sensitivity at every wavelength. To meet these requirements, proper optical filters are mounted before the silicon photodiode in the detector. This technique is complicated and the cost is high.
Besides the linear dynamic range, stray light level is another important parameter for multi-channel array spectrometers. Stray light is the radiation at wavelengths other than the one being measured, which enters the detector at the same time and attribute to measurement errors. This parameter is important for most applications, particularly in the following situations.
Line spectrum emissions have large empty regions in spectral distribution.
Spectrometer calibration is usually performed with standard illuminant A which exhibits 25 times more energy at 780 nm than at 380 nm. So during calibration, an instrument response at 380 nm will be over-estimated due to stray light.
Stray light can be caused by grating blemishes, dust on grating and mirror surfaces, high order light and re-entrant light caused by multi-reflection in the optical bench and so on. Good detail design can reduce stray light to a reasonable level, for example, by using a full-sealed structure, painting the inner of the instrument matt black and setting diaphragms in front of the collimating mirror and gratings, and so on. However, further improvements are needed for best accuracy. The simplest improvement is to introduce a turret of bandpass filter to attenuate spectral components well away from the tuned wavelength, and double monochromators are the best way to reject stray light. The filter turret and the double monochromators are mostly applied to scanning type instruments currently. However, they cannot be easily used for increasing accuracy of multi-channel array spectrometers. Thus, one of the limitations to the high speed array type arises from stray light response.
Filters including longpass filters, shortpass filters or bandpass filters are usually introduced into the high speed array spectrometer. The longpass filters are the most applied ones which are installed between a light entrance aperture and an entrance slit, behind the entrance slit, or on front of the array detector. The longpass filters are usually used to reject higher order responses. However, they have many shortages because by only using the longpass filters, the stray light caused by the multi-reflection in the optical housing can not be rejected. Also, there will be errors in calibration and measurement of above mentioned illuminant A like light which has lower intensity in short wavelength ranges. The shortpass filters, bandpass filters and linear variable filters (LVF) that are long-pass, short-pass and variable-bandwidth filters, are sometimes introduced in the multi-channel array spectrometers for spectrally shaping the excitation energy from broadband sources. They are usually installed on the front of the array detector. While these filters also have limitations, for example, because they are installed at the end part of the test optical path, the most important limitation is that they can not prevent producing of stray light as the broadband light transmitting in the optical housing.